This invention relates to an improved scintillation camera crystal, and more particularly to such a crystal with increased scattering and reduced light spreading.
Radionuclide emission scintillation cameras, also called Anger cameras, are used to image the distribution of gamma-ray emission radioactive material within a body part or organ, such as the brain, breast or heart, for example, for diagnostic purposes. A source of radiation is administered to the patient, which typically consists of a pharmaceutical tagged with a gamma-ray emitting radionuclide designed to go to and deposit in the organ or elements of the body under diagnostic examination, such as for example in the detection of a lung tumor. Gamma rays emitted by the radionuclide are received and detected by the camera, the position of each detected ray event is determined, and the image of the radioactivity distribution in the organ or other body part is constructed by known techniques from an accumulation of events.
Scintillation cameras usually employ an optically continuous scintillation crystal, such as thallium activated sodium iodide, NaI(Tl), sodium activated cesium iodide CsI(Na), thallium activated Cesium iodide CsI(Tl), and others as gamma-ray energy transducers. The energy of the gamma-rays are absorbed in the crystal and converted to light emissions called scintillation events, each light event having an energy proportional to the energy of the absorbed gamma-ray. In conventional cameras, light is transmitted from the crystal through a transparent interface (window) to an array of photosensors. The optical window may be rigid such as glass, which is coupled to the scintillator and the photosensors by silicon gel or optical grease, for example. The optical window may also be an optically transmitting liquid, which also directly couples the scintillator to the sensors.
The optically coupled array of photosensors, usually photomultipliers, or photodiodes, absorb and convert light to electrons by the photoelectric effect and the electrons are amplified by the sensor. Amplified signals generated from photosensors in the vicinity of the scintillation event are then mathematically combined by known analog or digital means to determine the position and the energy of the gamma-ray absorption in the crystal.
In general, the scintillation camera method works best with thin crystals so that the spread of the light transferring to a photosensor array is less than the width of a photosensor. In most high resolution cameras capable of detecting a high number of events per second (high count rate) it is preferable to detect a large fraction of the emitted light in a small number of photosensors, usually in clusters in the range of seven to nine, in the vicinity of the event. Increasing the spread of light beyond a minimum needed for good position resolution results in reduced count rate due to pulse pileup as well as poorer resolution, because light spreading to photosensors distal to the scintillation event inhibit the simultaneous position determination of spatially overlapping light spreads from multiple events. For thick crystals, e.g., 20 to 30 mm thick employing photomultipliers as photosensors, for example, light scintillations emitted as a consequence of the absorption of gamma-rays typically spread over much larger numbers of photomultipliers and, although various schemes have been developed to reduce the effect of pulse pileup these invariably result in poorer imaging characteristics by degrading both position and energy resolutions.
While crystals of about 10 mm thick, or smaller, are useful for detecting most of the so-called xe2x80x9csingle photonxe2x80x9d emitters, which typically have energies of 150 keV, thicker crystals are needed to detect high energy emissions with reasonable efficiency, as for example the pairs of 511 keV gamma-rays emitted as a result of positron annihilation. The efficiency of 10 cm thick scintillation crystals for the detection of pairs of annihilation gamma-rays, using coincidence detection means, is about 12% of that for detecting 150 keV single photons.
Another problem, more manifest in thicker crystals, is that gamma rays absorbed near the surface give rise to light spreads that are broader than rays that are absorbed deep into the crystal. As a result, both the energy and spatial resolutions of a camera, which registers the cumulative results from light emissions from all depths of absorption, is compromised unless corrections for depth of absorption can be made. This well-known phenomenon has been described by Gagnon, U.S. Pat. No. 5,576,546, in a disclosure of a method to determine the depth of interaction of gamma rays so as to correct for resulting resolution degradations.
It is therefore an object of this invention to provide improved scintillation camera crystal.
It is a further object of this invention to provide such a crystal which decreases spreading by increasing scattering of the light generated by the crystal.
It is a further object of this invention to provide in which the spread of light from a scintillation can be tailored as a function of the depth of the absorption in the crystal of the gamma ray generating the scintillation.
It is a further object of this invention to provide for narrowing the spread of light leaving the scintillation crystal in transit to photosensors of a scintillation camera.
It is a further object of this invention to provide a scintillation crystal with narrower light transfer functions to the photosensors of a scintillation camera.
It is further an object of this invention to provide thick scintillation crystal with improved light transfer functions.
It is a further object of this invention to provide improved gamma ray sensing efficiency even with thick crystals.
It is a further object of this invention to provide improved count rate for cameras using thick crystals.
It is further an object of this invention to provide improved energy discrimination for thick crystals.
It is further an object of this invention to provide improved spatial resolution for thick crystals.
This invention results from the realization that an improved scintillation crystal with less spreading can be achieved by using a plurality of holes in the crystal which contain some material different than the material of crystal to defect the light generated by the crystal and increase its scattering.
This invention features a scintillation camera crystal including a plurality of light scattering holes in the crystal extending toward the photosensor and communicating with at least one surface of the crystal. The crystal is formed by a first material and the holes include a second material which differs from the first material for deflecting the light generated by the scintillation crystal in response to incident gamma rays and reducing the spread of the generated light.
In a preferred embodiment, the holes may communicate with both surfaces of the crystal. The second material may refract the light. The second material may have a different index of refraction than the first material. The second material may be air. The second material may include a reflective coating. The holes may be arranged in a random pattern or a regular pattern. The regular pattern may include squares or triangles. The hole may be cylindrical. The holes may be tapered. They may be stepped. They may be inclined to the surfaces of the crystal. They may be perpendicular to the surface of the crystal. The holes may be right circular cylinders. At least one of the surfaces of the crystal may be grooved. Both of the surfaces of the crystal may be grooved. The grooves may form an array of pyramidical structures. The crystal surface toward the photosensors may be transmissive while the other surface of the crystal may be reflective. The holes may be arranged in a uniform density pattern or a non-uniform density pattern. The holes may uniform in size and/or shape. The crystal may be spherical, arcuate or planar. The crystal may be segmented.